1. Field of the Invention
This invention relates to a general method for creating robust, catalytically-active materials suitable for use in a variety of applications. The catalytically-active materials of this invention are engineered to resist attrition or to exhibit controlled rates of attrition in a variety of host environments. These applications include, but are not limited to, petroleum refining, Fischer-Tropsch syntheses, chemical synthesis and production, including the synthesis and production of pharmaceutical compounds, the production of plastics and foodstuffs, and catalysts that effect a chemical or physical change in combination with complexes of DNA-related molecules or living organisms, such as natural or genetically modified bacteria. This invention further relates to catalysts and catalytically-active materials suitable for use in gasification reactor vessels, in particular fluidized bed gasification reactor vessels, and combustion processes. Finally, this invention relates to a method and apparatus for reducing or eliminating tars, which are typically defined as organic compounds having a molecular weight equal to or greater than 78, for example, benzene, and other undesirable volatile compounds produced during the gasification of various feedstocks including coal, biomass and waste materials and the combustion of various fuels.
2. Description of Related Art
In general terms, gasification is a process whereby solid carbonaceous materials such as coal and biomass are converted into cleaner-burning gaseous fuels. Gasification is frequently carried out in a fluidized bed reactor, a reactor chamber comprising a fluidized bed support disposed within the reactor chamber and a fluidized bed material disposed on the fluidized bed support, which fluidized bed material comprises an inert component that is either fully inert or has low catalytic activity, and a catalytically-active component that is dispersed within or upon the inert component. During the gasification process, numerous by-products, including tars and other volatile materials, are also generated. Environmental regulations require that these by-products be treated or otherwise disposed of in an environmentally acceptable manner.
Catalysts are recognized as being essential for reducing or eliminating the tars that accompany the gasification of solid materials. Robust, efficient catalysts that are added to or comprise the bed material of fluidized bed gasifiers represent a significant development because they reduce the overall gasifier footprint by virtue of their incorporation into the gasifier, offer the possibility of substantially eliminating tar formation, and retain their activity in a harsh, chemically active environment. However, the development of in-bed catalysts has been slow because, to date, mineral geology has been relied upon for selection of the best materials for catalyst development. Thus, the ability to move away from earth mineralogy as the basis for identifying and selecting suitable materials is a highly desirable objective, opening the door to the development of new catalyst formations from present waste materials, such as arc furnace dust, mold sands, various slags and mill scale.
Catalytically-active materials employed for reducing or eliminating tars that are produced in the gasification of coal, biomass, or other materials, as well as for other applications, typically comprise two fundamental components, a catalytically-active component and a base or substrate component for support of the catalytically-active component. The base or substrate component is a material substantially physically and chemically inert to the environment in which it is to be used and is typically either a solid monolithic structure wherein the catalytically-active component is deposited onto the surface of the structure or a porous structure wherein the catalytically-active component is disposed on the surface of the structure and in the pores of the structure.
At the present time, most catalysts are prepared by depositing thin layers of catalytically-active materials onto rigid, attrition-resistant substrates or by coating rigid, refractory monoliths (typically used in a self-supporting off-bed tar-cracker or specialized support structure for chemical synthesis). Typical substrates include α-alumina and zirconia. The method of applying a catalytically-active layer onto an inert support varies, but generally two approaches are employed. The most common method, the incipient wetness or wet impregnation method, is typically accomplished by immersion of the substrate in an aqueous solution of a catalyst precursor (typically a metallic salt), resulting in a coated substrate, followed by heating of the coated substrate to convert the catalyst precursor to a catalytically-active material, typically a metallic oxide. If the substrate is porous, a so-called three-dimensional or 3-D catalyst is created. If the surface is not porous, a two-dimensional or 2-D catalyst is created.
Another recently developed method for preparing catalysts uses thermal plasma chemical vapor deposition or TPCVD. This method is primarily used to produce monolithic two-dimensional catalysts and involves spraying a concentrated solution of a metallic salt through a plasma torch onto a suitable refractory substrate. Thus, the end product is a catalyst comprising an inert, rigid substrate with a thin, catalytically-active outer layer. If the outer layer is damaged through attrition or fragmentation, overall catalytic activity is reduced. However, the advantage of this approach is that relatively large amounts of high surface area catalysts that incorporate precious metals can be produced with minimal amounts of these materials.
Two routes are generally available for employing catalysts to reduce or eliminate tars that are produced during the gasification of coal, biomass, or other materials. The first route is through the use of catalysts as described above disposed on the surface of otherwise inert monolithic substrates, which are disposed downstream of the gasification reactor vessel so that the gasification product gases are exposed to the catalysts. Typical of such catalysts are oxides of nickel, cerium, ruthenium, and lanthanum. Catalytic materials have also been embedded into ceramic candle filters so that during high temperature gas particle separation, intimate gas-catalyst contact is assured.
The second route is through the direct introduction of suitably small fragments or beads of catalytic materials into the bed of a fluidized-bed gasifier. These catalytically-active materials are either prepared by depositing a catalyst onto an inert, abrasion-resistant substrate, either monolithic or porous, or are available as naturally-occurring minerals that exhibit catalytic activity. Dolomite and olivine are examples of this type of naturally occurring material. When properly sized fragments of dolomite or olivine are added to the bed of a fluidized bed gasifier, they become intimately involved in the gasification process, achieve good contact with raw fuel gases and inhibit tar formation by cracking or reforming the tars as they are produced to generate lower molecular weight hydrocarbons and carbon. However, a long recognized problem with dolomite is that within the bed of a gasifier, dolomite is rapidly calcined. Calcined dolomite is friable and, thus, tends to be quickly milled within the bed until its particle size becomes too small to be retained within the reactor vessel. This creates the need to replace the attrited catalyst and produces undesirable waste particulate material, aside from ash, that must be separated from the fuel gas. Thus, there is a need for durable catalytic materials that can withstand fluidized bed temperatures and resist fragmentation or, at a minimum, abrade at a slow, predictable rate so that fresh catalyst remains available.
As previously stated, in addition to dolomite, olivine is a naturally occurring catalytic material suitable for reducing tars in fuel gas. Olivine, which is a very hard, attrition-resistant, glassy material which has a very high melting point (1760° C.) and which exhibits catalytic activity for tar removal with extended heat treatment in air at about 900° C., is actually a mixture of two minerals—Fe-rich fayalite (Fe2SiO4) and Mg-rich forsterite (Mg2SiO4). Untreated, naturally occurring olivine exhibits less activity for tar removal than dolomite. However, it has been found that heating olivine for extended periods in air at about 900° C. appears to provide sufficient mobility to iron within the olivine so that it becomes enriched at the olivine-air interface. Free iron at the olivine-air interface is then transformed into an oxide by reacting with oxygen in the air and olivine that has been prepared in this manner has been found to exhibit enhanced catalytic activity for reducing tars in biomass-derived fuel gas. In addition, the catalytic activity of olivine is further enhanced by calcining at 1100° C. olivine that has been treated with an aqueous solution of Ni(NO3)2·6H2O to a level of about 2.8 weight percent nickel content when dry. By virtue of this treatment, a very active olivine-based catalyst is produced that contains abundant quantities of NiO on the surface of finely divided olivine that has been sized to be in the range of about 250 μm to about 600 μm. Calcining at either higher or lower temperatures appears either to drive the NiO into the olivine or restrict adhesion of NiO to the surface of the olivine. This method of preparing a NiO-based catalyst on an olivine support is taught, for example, by International Patent Publication No. WO 01/89687 A1.